Disable scroll anchoring if the user engages in a drag motion

third_party/WebKit/Source/core/input/PointerEventManager.cpp

Index: third_party/WebKit/Source/core/input/PointerEventManager.cpp
diff --git a/third_party/WebKit/Source/core/input/PointerEventManager.cpp b/third_party/WebKit/Source/core/input/PointerEventManager.cpp
index 3fe3d505ceb762ad2aa1113063f61d54fca70961..5a18aae0841dae474c34bcf3d723dce1ebd0c350 100644
--- a/third_party/WebKit/Source/core/input/PointerEventManager.cpp
+++ b/third_party/WebKit/Source/core/input/PointerEventManager.cpp
@@ -153,6 +153,15 @@ WebInputEventResult PointerEventManager::dispatchPointerEvent(
         }
     }
 
+    if (pointerEvent->type() == EventTypeNames::pointermove
+        && m_pointerEventFactory.isActiveButtonsState(pointerEvent->pointerId())) {
+        // If we see a pointermove while a button is down, presumably the user
+        // is engaging in drag motion.
+        // TODO(ymalik): This will need to be updated to account for pointermove
+        // events that occur within the slop region (see crbug.com/632067).
+        m_frame->view()->setUserMayHaveDragged(true);

[Navid Zolghadr, 2016/08/04 18:11:00]

Maybe for the events that cannot possibly cause scroll you can do this on
pointerdown?
This field (which we haven't added yet and it is unrelated to the change for the
slop region) could be useful in that case. I don't think we have a crbug for
that thought. Just something to keep in mind.

+    }
+
     if (!RuntimeEnabledFeatures::pointerEventEnabled())
         return WebInputEventResult::NotHandled;
     if (!checkForListener || target->hasEventListeners(eventType)) {